This invention relates to a signal generator (color encoder) for a color television system. An object of the invention is an improvement in the picture quality when color encoded signals are reproduced in the picture displayed by a usual color television receiver.
FIG. 1 is a block diagram representing a flow of signal processing at a conventional encoder and a usual television receiver, wherein in a transmitter 1, primary color signals R, G and B are converted by a matrix circuit 3 into a luminance signal. The primary color signals are also given to a matrix circuit 4 to produce color-difference signals R-Y and B-Y, the color-difference signals being fed to a modulator 5 and modulated thereat to be output as a chrominance subcarrier. The chrominance subcarrier is passed through a band pass filter 6 and added to the luminance signal at an adder 7 to be fed to a transmission line 8 as a composite color TV signal. A receiver 2 separates the luminance signal and chrominance subcarrier from the composite color TV signal through a band pass filter 9 and feeds the luminance signal and chrominance subcarrier to an amplifier 10 and a modulator 11 respectively, the demodulator 11 demodulating the chrominance subcarrier to produce a color-difference signal. At a matrix circuit 12, the luminance signal output from the amplifier 10 is added to the color-difference signals R-Y, G-Y and B-Y to produce primary color signals R, G and B and send them to a color picture tube (not shown).
In the above-noted conventional color television system, the bandwidth of chrominance subcarrier is narrower than that of luminance signal. Especially at the receiver side, the bandwidth of the color difference signal obtained by the demodulation of the chrominance subcarrier is about 500 KHz, which is extremely narrow in comparison with the luminance signal bandwidth of 3 MHz or 4 MHz, resulting in the following defect in picture quality.
The defect is that brightness reproduced on the picture has distortion with respect to its original brightness. Such a phenomenon is caused by a cooperative effect of due to the fact that the color picture tube used for the receiver has a non-linear characteristic generally called a .gamma.-characteristic and due to the fact that the bandwidth of color-difference signal is narrow. In other words, if the color picture is high in color saturation, the brightness information reproduced on the picture is influenced by the receiver's .gamma.-characteristic and is transmitted from the color-difference signal channel as from the well as luminance signal channel. A narrow bandwidth color-difference signal channel cannot accurately reproduce the original brightness information at the picture edge of a saturated color, and the brightness often decreases.
Such phenomenon is exemplarily shown in FIG. 2, in which the luminance actually reproduced on the picture tube in response to transient signals of so called color bar is illustrated. The axis of ordinate in FIG. 2 represents relative luminance to the normalized white luminance level, and the axis of abscissa a time period, only a transient response period being shown. As seen from FIG. 2, the transient signal of color bar generally produces a "luminance trough". At the transient period from the green to magenta, the "luminance trough" is particularly larger. This is the reason for a readily appearing dark part at the boundary of green and magenta when the color bars are displayed by the receiver. Such a luminance distortion which is formed during the transient period of a color picture is a defect in the conventional color television system.